Insulated tools, test instruments and safe testing

Testing and troubleshooting are the energized tasks that qualified workers perform most often, and they are behind a large share of arc flash incidents. A meter with the wrong rating, a damaged test lead, a probe that slips between phases or a meter left on the current range can turn a routine measurement into a fault. This lesson covers how test instruments are rated, how to choose and inspect them, how insulated and insulating tools differ, and the work practices that make testing safer.

Overvoltage installation categories

Electrical systems carry not only their nominal voltage but also brief transient overvoltages, caused by lightning, switching of large loads and faults. These transients can be thousands of volts and, if they exceed what a meter can withstand, they can cause an internal flashover that becomes an arc in the worker's hands. The closer a location is to the utility supply, the higher the available fault current and the more energetic the transients. IEC 61010-1, adopted in Canada as CSA C22.2 No. 61010-1 and in the United States as UL 61010-1, classifies locations into measurement categories:

• CAT I: electronic equipment and circuits not directly connected to the mains, such as protected low-energy circuits.
• CAT II: receptacle-connected loads, such as appliances, portable tools and outlets more than about 10 m (30 ft) from a CAT III source.
• CAT III: the distribution level inside a building: panelboards, distribution boards, motor control centres, feeders, bus duct and fixed equipment such as large motors.
• CAT IV: the origin of the installation: the service entrance, utility meter, service conductors and outdoor overhead or underground lines feeding a building.

The rating marked on a meter combines a category and a voltage, such as CAT III 1000 V or CAT IV 600 V. Both matter. A higher category at a given voltage, or a higher voltage in the same category, means the meter is designed to withstand larger transients. A meter must be rated for the highest category and voltage where it will be used.

Meter features that matter

Beyond the category rating, several features separate a meter that is suitable for industrial work from one that is not:

• Third-party certification to IEC 61010-1 (CSA C22.2 No. 61010-1 or UL 61010-1) by a recognized testing laboratory, shown by its mark on the meter. A printed CAT rating without certification is a claim, not proof.
• High-energy fuses on the current inputs, rated for the voltage and with an interrupting rating high enough for industrial fault levels, such as 1,000 V with an interrupting rating of 10 kA to 20 kA or more. Cheap meters may have glass fuses that cannot safely interrupt an industrial fault.
• Input protection and warnings, such as an alarm if test leads are in the current jacks while the dial is set to voltage.
• Test leads with shrouded connectors, finger guards and probe tips that can be fitted with covers so that no more than about 4 mm of metal is exposed, as required for CAT III and CAT IV use by IEC 61010-031.
• A low-impedance voltage function, which helps distinguish real voltage from ghost voltage induced on de-energized conductors.
• A design that allows the meter to be hung or held so that the worker's hands stay as far from the circuit as possible.

Inspecting test equipment

CSA Z462 requires test instruments, equipment and their accessories to be rated for the circuits and equipment where they will be used, designed for the environment, and visually inspected for external defects and damage before each use. If there is a defect or evidence of damage, the item must be removed from service. The same standards require the operation of a test instrument used to verify absence of voltage to be confirmed before and after use (the live-dead-live check in Lesson 7).

A quick pre-use check includes the meter case for cracks, the display and dial for correct function, the battery indicator, the lead insulation for cuts, nicks, melted spots and exposed conductor, the connectors for damage, the probe tips for wear and the tip guards. Many meters have a fuse test function; use it. Leads are consumable items and should be replaced whenever there is doubt.

Insulated and insulating tools

Two kinds of hand tools are used near energized parts. Insulated tools are metal tools with an insulating covering, such as screwdrivers, pliers, wrenches and cable cutters, tested and marked for live working up to 1,000 V AC and 1,500 V DC. They are made to IEC 60900 or ASTM F1505 and carry the double triangle symbol with the voltage rating. Insulating tools are made mostly of insulating material, with metal only where it is needed at the tip, and are used for tasks such as positioning conductors. Both are for use only with the rest of the required PPE and only within their ratings.

Insulated tools must be inspected before each use. If the insulation is cut, chipped, cracked or worn to the point where the inner coloured layer shows (many tools have a two-colour coating for this reason), the tool must be removed from service. Electrical tape wrapped around an ordinary screwdriver does not make it an insulated tool.

For medium and high voltage, live-line tools such as hot sticks and shotgun sticks, made of fibreglass-reinforced plastic to ASTM F711, allow qualified workers to operate switches, install grounds and test from a distance. They must be kept clean and dry, inspected before use and periodically dielectric-tested.

Safe testing practices

Good practice for testing energized circuits combines the right equipment with habits that limit the chance of a slip:

• Plan the measurement. Know the expected voltage and the points to be tested before opening anything.
• Set the meter to the correct function and range before connecting, and check the lead positions every time.
• Use the one-hand rule where practical: connect the ground or neutral lead first, keep the other hand away from the equipment, and remove the hot lead first.
• Use probe tip covers, and where possible use test points, finger-safe terminals or test blocks that avoid exposing bus.
• Position yourself to the side of the enclosure opening, not directly in front of it, and keep your face out of line with the opening where possible.
• Do not lean on the enclosure or allow your body to contact grounded metal while probing.
• Stop if the readings are unexpected. A voltage where none should exist, or none where some should, means the circuit is not what you thought.
• Use remote displays, wireless meters and permanently installed voltage indicators where available to take readings without opening equipment.

Ghost voltage and other surprises

A high-impedance digital meter can show a voltage on a de-energized conductor that is running near energized ones, because capacitive coupling induces a small voltage that the meter's high input impedance does not load down. This ghost voltage can be several tens of volts. A low-impedance (LoZ) setting or a dedicated tester applies a load that collapses ghost voltage while showing real voltage. The difference matters: treating a real voltage as a ghost could be fatal, while treating a ghost as real leads to confusion. When in doubt, treat the conductor as energized until the source is identified.

Proving units and calibration

A tester used to verify absence of voltage must be proven before and after each use. A known live source nearby is acceptable, but it may be far from the point of work or at a different voltage. A portable proving unit, a small battery-powered device that generates a known test voltage, allows the live-dead-live check to be done at the point of work. Proving units must themselves be checked and their batteries maintained. Where a proving unit generates a voltage lower than the system voltage, the employer's procedure should confirm that it is adequate for the tester and the task.

Test instruments should also be calibrated and functionally checked at the intervals the manufacturer recommends, and whenever they have been dropped, exposed to a fault or show inconsistent readings. Calibration labels show when the next check is due. An instrument out of calibration may still indicate the presence of voltage, but it can no longer be relied on for the measurements made during troubleshooting.

Finally, keep test equipment clean and dry, store leads loosely coiled rather than tightly wrapped, and never use a meter that has been exposed to water, solvents or conductive dust until it has been inspected.